In modern life, automobile is gradually becoming an indispensable tool. However, the power of automobile derives substantially from the fossil oil which is limited. At the same time, the rapid development of the automobile industry also encounters the pressure of reducing carbon dioxide emission. Therefore, the demand of reducing vehicle fuel consumption has become more and more urgent. By reducing fuel consumption, not only vehicle operating cost but also carbon dioxide emission can be reduced, and the stress of oil resource can be relieved. Besides design factors of automobiles, the rolling resistance of tire is also an important factor influencing the vehicle fuel consumption. The fuel consumption caused by tire rolling resistance comprises 14-17% of total vehicle fuel consumption. It is generally believed that the fuel consumption may be reduced by a factor of 1-2% relative to per 10% reduction in tire rolling resistance. Thus, reducing tire rolling resistance is regarded as one of the most important measures for reducing fuel consumption.
However, thorny problems have been encountered in the research for reducing the rolling resistance of tire rubber material (mainly tread rubber), i.e. the so-called “magic triangle” problem in which rolling resistance, wet skid resistance and wear resistance are mutually restricted. Simply increasing the amount of the softener can improve the wet skid resistance of tire, but wear resistance decreases and rolling resistance increases. Increasing the amount of reinforcing filler (carbon black or silica) can reduce rolling resistance to some extent, but the reinforcing filler is difficult to be uniformly dispersed in rubber which may lead to the deterioration of wet skid resistance. Increasing the amount of vulcanizing agent (i.e. increasing crosslinking density) leads to the same effect as obtained upon increasing the amount of reinforcing filler, i.e. reducing rolling resistance while deteriorating wet skid resistance. In order to achieve the balance of the above three properties, besides the attempt of optimizing the designs of tire structure, extensive studies have been carried out worldwide on the formulation of rubber (mainly tread rubber). On one hand, efforts are focused on synthesizing suitable rubber raw materials such as solution polymerized styrene-butadiene rubber (SSBR), transpolyisoprene (TPI), styrene-isoprene-butadiene rubber (SIBR), high vinyl butadiene rubber (HVBR) etc. On the other hand, efforts have been paid on finding modifiers and practical formulations with better comprehensive performances. Some progresses have been achieved in the formulation research. Representative examples include the combination of solution polymerized styrene-butadiene rubber (SSBR) etc. with carbon black and silica or inversion carbon black system. This system is characterized by substantially fixed main formulation with only variable reinforcing filler, and by simplicity of industrialization. The disadvantages of this system lie in that more silane coupling agents and heavy equipment load are required during the compounding process, and the wear resistance of the vulcanized rubber is not satisfactory.
The rubber gels produced by direct polymerization process or chemical crosslinking process using peroxides may improve the properties of vulcanized rubber if properly formulated. For example, European patent EP405216 and German patent DE4220563 respectively report that the wear resistance and temperature rise by fatigue of the vulcanized rubber were improved by adding neoprene rubber gel or butadiene rubber gel into the rubber composition respectively. However, the wet skid resistance decreases.
Therefore, many patents started to improve the properties of vulcanized rubber by using modified rubber gel. For example, a surface-modified butadiene rubber gel and styrene-butadiene rubber gel were used in U.S. Pat. No. 6,184,296 (the latex particles in the gel has a swelling index of 4-5, and a particle size of 60-450 nm). As a result, the rolling resistance of the vulcanized rubber of natural rubber (NR) formulation system was reduced without any deterioration in strength properties.
In U.S. Pat. No. 6,133,364, chloromethyl styrene was grafted onto the surface of styrene-butadiene rubber gel, and then the modified rubber gel was used in a NR formulation system. As a result, the rolling resistance of the vulcanized rubber was reduced and wet skid resistance is improved.
In U.S. Pat. No. 6,207,757, a chloromethyl styrene modified styrene-butadiene rubber gel was used to achieve the effect of lowering the rolling resistance of the vulcanized rubber in NR formulation system, and meanwhile, improving the wet grip and maintained longevity of tire.
In U.S. Pat. No. 6,242,534, styrene-butadiene rubber gels containing respectively carboxylate and amino group were used together in a NR formulation system. The rolling resistance of the vulcanized rubber system was reduced and the wet skid resistance was enhanced, while the stress at a given elongation was significantly increased.
In European patent EP1431075, a styrene-butadiene rubber gel and a plasticized starch were used to improve the properties of a silica system comprising a combination of styrene-butadiene rubber (SBR) and butadiene rubber (BR). As a result, wear resistance was improved, rolling resistance was reduced, and the specific gravity of the vulcanized rubber was low.
In U.S. Pat. No. 6,699,935, copolymerization modified styrene-butadiene rubber gel was used for conferring low rolling resistance as well as excellent wet skid resistance and wear resistance on the modified styrene-butadiene rubber formulation system.
The rubber gels mentioned in the patent references described above are all crosslinked by chemically crosslinking processes requiring both expensive crosslinking monomers and high energy consumption, and relating mainly to the natural rubber formulation system or silica system of the styrene-butadiene rubber and modified styrene-butadiene rubber formulation system. What is important is that the simultaneous improvements in rolling resistance, wet skid resistance and wear resistance can be obtained only after the crosslinked rubber gel has been modified. Although some of these patents disclose the particle size of the rubber gels, neither of them discloses whether or not a dispersion with initial primary particle size can be realized and whether or not a modification effect via the nano-scale rubber gel can be really achieved when these rubber gels are dispersed into the vulcanized rubber.